The present invention relates to inserts for use within the vane cavity of a nozzle segment and particularly relates to a method of connecting the nozzle vane cavity insert and nozzle one to the other.
In current gas turbine designs, nozzle segments are typically arranged in an annular array about the rotary axis of the turbine. The array of segments forms outer and inner annular bands and a plurality of vanes extend between the bands. The bands and vanes define in part the hot gas path through the gas turbine. Each nozzle segment comprises an outer band portion and an inner band portion and one or more nozzle vanes: extend between the outer and inner band portions. In current gas turbine designs, a cooling medium, for example, steam, is supplied to each of the nozzle segments. To accommodate the steam cooling, each band portion includes a nozzle wall in part defining the hot gas path through the turbine, a cover radially spaced from the nozzle wall defining a chamber therewith and an impingement plate disposed in the chamber. The impingement plate defines with the cover a first cavity in one side thereof for receiving cooling steam from a cooling steam inlet. The impingement plate also defines along an opposite side thereof and with the nozzle wall a second cavity. The impingement plate has a plurality of apertures for flowing the cooling steam from the first cavity into the second cavity for impingement cooling the nozzle wall. The cooling steam then flows radially inwardly through one or more cavities in the vane(s), certain of which include inserts with apertures for impingement cooling the side walls of the vane. Cooling steam then enters a chamber in the inner band portion and reverses its flow direction for flow radially outwardly through the impingement plate for impingement cooling the nozzle wall of the inner band. Spent cooling medium flows back through a cavity in the vane to an exhaust port of the nozzle segments.
In past designs, great difficulty has been encountered in inserting the insert into the nozzle cavity in a manner establishing an interface with the nozzle sufficient to provide a ready and easy securement to the nozzle, i.e., to provide an insert and nozzle casting with required tolerances to effect an interface facilitating brazing or welding the parts to one another. For example, and in current nozzle designs, the inserts have a band added to one end which is used to connect to the nozzle. One such design has a collar which attaches to the nozzle side wall band on top of a boss around an airfoil cavity. A second typical nozzle design has a flash rib cast into an airfoil cavity which serves as a connection point for the insert collar. When an insert has a collar on the end which enters the airfoil cavity first, this creates a significant clearance problem when inserting the insert. A secondary problem is forming the collar on the end of the complex three-dimensional shape of the insert. Further, it is highly desirable to have very tight tolerances on the collar end of the insert such that it can be brazed or welded to the nozzle. This becomes quite difficult with the addition of the collar on the end of the insert, both of which are formed of flexible sheet metal. During assembly of the latter design, the inserts also and inevitably have to have collars modified by hand to fit into the nozzle. With the poor tolerances of the collar-to-nozzle connection, the joint likewise becomes very poor. Further, the collar is too stiff to form it to the shape of the nozzle flash rib, so a large gap may result. As an example of the poor tolerances of the collar-to-nozzle connection, it will be appreciated that the gap between the collar and nozzle should be about 5 mils to provide a brazed joint. However, from a manufacturing standpoint, the collar and nozzle interface tolerance can be xc2x115 mils. Thus, the gap between the collar and nozzle is problematical, virtually impossible to braze without manual handling to achieve an approximate 5 mil gap and, from a manufacturing standpoint, not repeatably reproducible.
In accordance with a preferred embodiment of the present invention, the insertability of the inserts and the robustness of the joint connection between the inserts and the nozzle are significantly improved. Additionally, the repeatable manufacturability of the inserts is likewise improved. To accomplish the foregoing, the nozzle has a rib added to the casting sized to correspond to the desired impingement cooling flow gap between the insert and the interior nozzle wall. The sheet metal insert is formed without an ancillary collar about the end of the insert to be attached to the nozzle. By inserting the insert into the cavity with the open end first and from the opposite end of the cavity, the insert is received about the nozzle rib. Preferably, the insert is extended into the cavity such that the insert end lies slightly beyond the nozzle rib. The end of the insert that interfaces with the rib can then be formed to tightly fit the interface. This is accomplished by handworking or by using a mandrel, thus effectively swaging the insert end about the margin of the rib. By then slightly retracting the insert, the gap is reduced and the insert can be brazed or seam-welded about its edge to the nozzle rib. The foregoing described process substantially reduces the cost of the insert in comparison with prior methods as substantial time, effort and labor was previously spent attempting to manufacture the collars, insert the insert into the assembly and then weld the collars to the nozzle.
In a preferred embodiment according to the present invention, there is provided in a gas turbine, a nozzle segment having outer and inner bands, at least one of the bands including a nozzle wall defining a part of a hot gas path through the turbine, at least one vane extending between the bands in the hot gas path, a wall of the vane defining at least one cavity extending through the vane, an insert in the cavity spaced from the wall of the vane and having apertures for flowing a cooling medium onto the wall defining the cavity, a method of securing the insert in the cavity, comprising the steps of forming a rib about the cavity wall adjacent one of the inner and outer bands leaving an opening through the rib, inserting the insert into the cavity, subsequent to step (b), forming an end of the insert into substantial conformance with the opening through the rib and brazing the formed end of the insert and the rib to one another.